Ankle prosthesis for the arthrodesis of the calcaneum

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to prior French Application No. 06 09001, filed Oct. 13, 2006, entitled ANKLE PROSTHESIS FOR THE ARTHRODESIS OF THE CALCANEUM.

FIELD OF THE INVENTION

The present invention relates to a prosthetic ankle assembly and to a surgical method for implanting such an assembly, and in particular, to the treatment of an ankle in which the articulation between the astragalus and the calcaneus are damaged.

BACKGROUND OF THE INVENTION

One possible treatment involves fusing the three bones represented by the calcaneus, astragalus and tibia, as is proposed in US-A-2005/0107791. A long compression screw internally connects the tibia and the calcaneus by passing through the astragalus, in such a way as to block these three bones relative to one another. This results in a considerable loss of mobility for the patient, since the ankle is totally blocked.

Another possible treatment involves fusing the calcaneus and astragalus, with the aid of a dedicated arthrodesis rod and re-establishing the articular capacity between the astragalus and tibia, by implanting an ankle joint prosthesis. However, the two articulations then have to be treated in two stages: the arthrodesis of the calcaneus and astragalus requires a consolidation period of at least six months, which delays the implantation of the articular prosthesis between the astragalus and the tibia.

US-A-2005/0288792 discloses an astragalar component of an ankle joint prosthesis with which it is possible to re-establish the articular capacity between the astragalus and the tibia, while at the same time using an astragalocalcanean rod that extends from a lower side of the astragalar component to the upper part of the calcaneus, in so doing passing right through the astragalus. In one embodiment, the upper end of this rod is screwed directly into internal threads of the astragalar component, which threads open out on the lower side of this component. Before being implanted, the component and the rod are firmly immobilized relative to each other by screwing. When this screwed assembly is implanted, the rod guarantees good anchoring of the astragalar component, since the latter is thus fixed both to the astragalus and also to the upper part of the calcaneus, which explains why the prosthetic assembly is more specifically intended for revision of an arthroplasty of the ankle. However, the assembly does not effectively block the articulation between the calcaneus and the astragalus, since relative movements around and/or along the rod remain possible and, in practice, appear quickly.

FR-A-2 220 235 proposes a prosthetic ankle assembly comprising an astragalar component of an ankle joint prosthesis in the form of a support designed to be fixed to the astragalus of a patient. This support is intended to cooperate in an articulated manner with a tibial rod, with interposition of an intermediate articulated component. In its lower part, the astragalar support is joined to three anchoring feet, namely two anterior feet and one posterior foot, which are intended respectively to be inserted into corresponding cavities formed in the astragalus and in the calcaneus. As regards the posterior foot, one embodiment entails this foot being formed by at least one sagittal screw that passes through the support from top to bottom, until bearing with its head on the support, while its stem is inserted, without being screwed, into the cavity of the calcaneus, plugged with cement. The benefit of using such a screw is that, if necessary, it can be unscrewed in order to facilitate the removal of the support. Thus, the posterior foot of this prosthetic assembly provides a stable means of bearing on the calcaneus, but, even in the form of a screw inserted into cement, this foot does not totally block the articulation between the astragalus and the calcaneus in an effective manner.

WO-A-01/30264 and US-A-2005/124995 for their part disclose a prosthetic ankle assembly comprising an astragalocalcanean rod that is introduced into a cavity formed beforehand through the astragalus and in the upper part of the calcaneus. This rod is used to anchor an astragalar plate, which is fitted in place and fixed to the astragalocalcanean rod after the rod has been placed in the aforementioned bone cavity. Hence, this rod does not provide any effective blocking of the articulation between the calcaneus and the astragalus so that, in order to limit the relative movements between these two bones, additional screws are fitted between the astragalar plate and the astragalocalcanean rod, in order to stabilize the position of the latter.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a method and apparatus that blocks the articulation between the calcaneus and the astragalus and to re-establish an articular capacity between the astragalus and the tibia of a patient, all in a single intervention.

To this end, the subject matter of the invention is a prosthetic ankle assembly, comprising an astragalar component which forms part of an ankle joint prosthesis and which is designed to be fixed to the astragalus of a patient. An astragalocalcanean rod of which a first longitudinal end is designed to be fixed to the astragalar component, by its shape complementing that of a dedicated part of the astragalar component. The dedicated part is provided on a lower side of this component intended to be directed towards an upper face of the astragalus. The astragalocalcanean rod is equipped, at its second longitudinal end, with an engagement feature for osseous engagement of the calcaneus of the patient, so as to approximate and thereby compress the calcaneus against the astragalus when the first end of the rod is fixed to the dedicated part of the astragalar component.

When the prosthetic assembly according to the invention is implanted, its astragalar component permits articular movements between the astragalus and the tibia of a patient, especially by being connected to a prosthetic tibial component fixed to the tibia, and to a prosthetic bearing interposed between the astragalar and tibial components. On the other hand, the astragalocalcanean rod blocks the articulation between the calcaneus and the astragalus, by these two bones being compressed against each other. In practice, this bone compression is obtained by fixing the first end of the rod in the astragalar component, which engages the second end of the rod directly with the calcaneus. For this purpose, a fixation means and the engagement means provided respectively at the ends of the rod are structurally configured to obtain a progressive compression effect between the calcaneus and the astragalus as the second end of the rod advances into the calcaneus, whilst the first end is coupled to the dedicated part of the astragalar component resting on the astragalus.

In practice, the rod is introduced from a lower face of the calcaneus into an astragalocalcanean channel that has been drilled beforehand through the calcaneus and the astragalus and that opens out on the dedicated part of the astragalar component when the latter is placed on an upper face of the astragalus. Thus, advantageously in a single surgical intervention, the astragalar component and the astragalocalcanean rod are implanted by being coupled to each other during the intervention, while the rod passes right through the calcaneus and the astragalus, in order to compress them and thereby immobilize them against each other. In other words, the operation of arthroplasty between the astragalus and the tibia and the operation of arthrodesis between the astragalus and the calcaneus are performed during the same surgical intervention, which limits the surgical risks faced by the patient and which allows the surgeon to use the same surgical routes for the two operations.

In addition, the astragalocalcanean rod ensures immobilization of the astragalar component relative to the astragalus and to the calcaneus, which improves the fit of the astragalar component. Moreover, in the event of revision of the prosthetic assembly according to the invention, the bearing action on the calcaneus is safe and reliable in the sense that this bone is firmly fused to the astragalus.

According to other advantageous characteristics of this prosthetic ankle assembly, taken either singly or in all the technically possible combinations: <ul id="ul0001" list-style="none"><li id="ul0001-0001" num="0000"><ul id="ul0002" list-style="none"><li id="ul0002-0001" num="0014">the astragalocalcanean rod is designed to extend in length from the upper face of the astragalus, on which the astragalar component is fixed, as far as a lower face of the calcaneus directed away from the astragalus;</li><li id="ul0002-0002" num="0015">the engagement means comprises an external thread designed to engage in the bone substance of the calcaneus when the first end of the astragalocalcanean rod is fixed in the dedicated part of the astragalar component;</li><li id="ul0002-0003" num="0016">the engagement means comprises a ring, fixedly or moveably surrounding the second end of the astragalocalcanean rod and adapted to be pressed against a lower face of the calcaneus, opposite to the astragalus, when the first end of the rod is fixed to the dedicated part of the astragalar component;</li><li id="ul0002-0004" num="0017">the engagement means for engaging the calcaneus has a maximum transverse dimension strictly greater than the maximum transverse dimension of a fixation means for fixing the first end of the astragalocalcanean rod to the astragalar component;</li><li id="ul0002-0005" num="0018">either the first end of the astragalocalcanean rod or the dedicated part of the astragalar component is equipped with an external thread, while the other of them forms a substantially complementary internal thread;</li><li id="ul0002-0006" num="0019">the astragalar component is equipped, on its lower side, with an anchoring means for osseous anchoring in the astragalus, inside of which anchoring means the dedicated part is arranged;</li><li id="ul0002-0007" num="0020">the anchoring means includes a hollow stub in which the external thread or internal thread of the astragalar component is delimited, preferably in a manner substantially coaxial to the rest of the stub;</li><li id="ul0002-0008" num="0021">the astragalocalcanean rod is equipped, at its second longitudinal end, with a driving profile, preferably driven in rotation on itself, designed to cooperate with a tool for coupling the rod to the astragalar component;</li><li id="ul0002-0009" num="0022">the profile or another profile for driving the rod is designed to cooperate with a tool for disconnection between the rod and the astragalar component;</li><li id="ul0002-0010" num="0023">the assembly also comprises a tibial component of an ankle joint prosthesis, designed to be fixed to the tibia of the patient, and a prosthetic bearing designed to be interposed between the astragalar component and tibial component.</li></ul></li></ul>

The invention also relates to a surgical method for implanting a prosthetic ankle assembly, the assembly comprising an astragalar component of an ankle joint prosthesis, and an astragalocalcanean rod of which a first of its two longitudinal ends is designed to be fixed to the astragalar component, by its shape complementing that of a dedicated part of the component, and of which the second longitudinal end is equipped with an engagement means of osseous engagement. The method of implantation comprising steps of:

preparing an upper face of an astragalus to receive an astragalar component;

forming an astragalocalcanean channel between an the upper face of the astragalus and a lower face of a calcaneus;

locating the astragalar component on the upper face of the astragalus;

inserting an astragalocalcanean rod into the astragalocalcanean channel from the lower face of the calcaneus;

engaging a first end of the astragalo-calanean rod with the astragalar component and a second end of the astragalo-calanean rod with a lower region of the calcaneus; and

adjusting a distance between the second end of the astragalocalcanean rod relative to a lower surface of the astragalar component to adjustably compresses the calcaneus against the astragalus.

This method makes it possible, in a single surgical intervention, to implant a prosthetic ankle assembly of the kind defined above, in order to permit arthrodesis of the calcaneus and astragalus and, at the same time, arthroplasty between the astragalus and the tibia of a patient. Arthrodesis refers to a surgical fixation of a joint, ultimately resulting in bone fusion. Basically, the procedure is artificially induced ankylosis performed to relieve pain or provide support in a diseased or injured joint.

According to an advantageous embodiment of this method, during step i), a substantially cylindrical recess is hollowed out in the upper face of the astragalus in order to receive a hollow bone-anchoring stub which is provided on the astragalar component and in which the dedicated part is arranged, and, during step ii), the astragalocalcanean channel is drilled from the upper face of the astragalus, in a continuation of the recess.

In this case, to make matters easier, a drill bushing is used to drill the astragalocalcanean channel, by resting this bushing in the recess formed during step i).

Advantageously, in order to promote the fusion of the calcaneus and the astragalus, the method comprises, before step ii), an additional surgical step during which bone substance is inserted between the astragalus and the calcaneus by arthroscopy.

Moreover, after consolidation of the arthrodesis of the calcaneus and astragalus, the method advantageously comprises a postoperative step in which the astragalocalcanean rod is withdrawn by disconnecting its first end from the dedicated part of the astragalar component.

BRIEF DESCRIPTIONS OF THE SEVERAL VIEWS OF THE DRAWING

The invention can be better understood on reading the following description given purely by way of example and made with reference to the drawings, in which:

<figref idref="DRAWINGS">FIG. 1</figref> is a front view, in partial cross section, of a prosthetic ankle assembly implanted in the left ankle of a patient in accordance with an embodiment of the present invention;

<figref idref="DRAWINGS">FIG. 2</figref> is an exploded side view, according to the arrow II indicated in <figref idref="DRAWINGS">FIG. 1</figref>, of part of the prosthetic assembly, depicted without the bones of the ankle;

<figref idref="DRAWINGS">FIGS. 3A and 3B</figref> are schematic cross sections, along the line III-IIi in <figref idref="DRAWINGS">FIG. 1</figref>, illustrating two successive steps in the implantation of the prosthetic assembly; and

<figref idref="DRAWINGS">FIG. 4</figref> is a view similar to <figref idref="DRAWINGS">FIG. 3B</figref>, illustrating a variant of the prosthetic ankle assembly in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

<figref idref="DRAWINGS">FIG. 1</figref> shows schematically the lower ends of a tibia T and of a peroneal bone P or fibula of a human being, and also the corresponding astragalus A and calcaneus C, thereby illustrating the constituent bones of the left ankle of a patient. <figref idref="DRAWINGS">FIG. 1</figref> also shows a prosthetic ankle assembly <b>1</b> comprising four separate components implanted in the ankle of the patient, namely a tibial component <b>10</b>, an astragalar component <b>20</b>, a prosthetic bearing <b>30</b>, interposed between the tibial and astragalar components, and an astragalocalcanean rod <b>40</b>. In <figref idref="DRAWINGS">FIG. 1</figref>, only the bearing <b>30</b> is shown in frontal section, whereas the broken lines are used to symbolize the contours of the components concealed by bone substance.

For convenience, the description that follows relates to the bones of the ankle in their anatomical position, that is to say the terms “posterior” or “rear”, “anterior” or “front”, “upper”, “lower”, etc., are to be understood in relation to the ankle of the patient when standing upright on a substantially horizontal surface.

The tibial component <b>10</b> comprises a plate <b>11</b> which is attached in a fixed manner to the lower end of the tibia T. For this purpose, the plate <b>11</b>, on its upper face <b>11</b>A, is joined, via a sagittal fin <b>12</b>, to a hollow bone-anchoring stub <b>13</b>. Additional and/or alternative means for anchoring the plate <b>11</b> are conceivable, provided that they effectively immobilize the tibial component at the lower end of the tibia T.

On its lower side, the plate <b>11</b> delimits a plane surface <b>11</b>B intended to form a sliding support for the plane upper surface <b>30</b>A of the bearing <b>30</b>.

The astragalar component <b>20</b> comprises a main block <b>21</b> attached in a fixed manner to the upper end of the astragalus A. For this purpose, the block <b>21</b> is made integral with a hollow bone-anchoring stub <b>22</b> that extends downwards from the lower side <b>21</b>B of the block <b>21</b>, as is represented by broken lines in <figref idref="DRAWINGS">FIG. 1</figref> and as can be clearly seen in <figref idref="DRAWINGS">FIG. 2</figref>.

On its upper side, the block <b>21</b> delimits an articular surface <b>21</b>A intended to cooperate with a matching articular surface <b>30</b>B delimited by the lower side of the bearing <b>30</b>. In sagittal section, the surface <b>21</b>A has an arched profile, with its concavity directed downwards, as can be seen in <figref idref="DRAWINGS">FIG. 2</figref>. The articular surfaces <b>21</b>A and <b>30</b>B are thus designed to slide against each other along this curved profile, in what is an essentially antero-posterior direction. Advantageously, these surfaces <b>21</b>A and <b>30</b>B between them permit a medio-lateral clearance, which combines with the curved movement of sliding in the antero-posterior direction.

As is shown in <figref idref="DRAWINGS">FIG. 2</figref>, the rod <b>40</b> extends lengthwise about a completely vertical central axis X-X. This rod comprises, in succession along its length, a first threaded end <b>41</b>, an externally smooth and regular part <b>42</b>, and a second threaded end <b>43</b>. The end <b>41</b>, the regular part <b>42</b> and the end <b>43</b> are successively of increasing size, such that, on the one hand, the maximum diameter d₄₁of the external thread <b>41</b>A of the end <b>41</b>, that is to say the maximum transverse dimension of this end <b>41</b> in the area of the crest of the thread, is less than or equal to the external diameter of the part <b>42</b>, and, on the other hand, the maximum diameter d₄₃of the external thread <b>43</b>A of the end <b>43</b>, that is to say the maximum transverse dimension of this end <b>43</b> at the crest of the thread <b>43</b>A, is strictly greater than the diameter of the regular part <b>42</b>.

The external thread <b>41</b>A of the end <b>41</b> is designed to be received by screwing into a complementary internal thread <b>22</b>A which, as is indicated by broken lines in <figref idref="DRAWINGS">FIG. 2</figref>, is delimited by the inner face of the hollow stub <b>22</b> and is preferably centred on the central longitudinal axis of this stub.

At its end <b>43</b>, the rod <b>40</b> delimits a recess <b>44</b> which is substantially centred on the axis X-X and opens out axially downwards to the outside, as is indicated by broken lines in <figref idref="DRAWINGS">FIG. 2</figref>. This recess <b>44</b> has a transverse profile <b>44</b>A, for example of square shape, intended to cooperate with a complementary tool for driving the rod <b>40</b> in rotation on itself about the axis X-X.

A surgical method for implanting the prosthetic ankle assembly <b>1</b> will now be described.

In a first step, the bones of the patient's ankle have to be prepared. For this purpose, the surgeon uses cutting means (not shown) in such a way as to, on the one hand, resect the lower end of the tibia T and form a substantially plane surface T₁, and, on the other hand, resect the upper end of the astragalus A and form a multi-facet surface A₁comprising two main surfaces which are inclined relative to each other, as can be seen in <figref idref="DRAWINGS">FIG. 3A</figref>. In the face A₁, a cylindrical recess A₂, indicated by broken lines in <figref idref="DRAWINGS">FIG. 3A</figref>, is hollowed out in an overall vertical direction, for example by means of a milling cutter. This recess A₂thus defines a central longitudinal axis A₃.

At this stage of the surgical intervention, the upper face A₁of the astragalus A is in a configuration suitable for receiving the astragalar component <b>20</b>, it being understood that the recess A₂is made substantially complementary to the stub <b>22</b> in order to anchor this component <b>20</b> on the astragalus. However, before fitting the astragalar component <b>20</b> in place, the surgeon drills through the astragalus A and calcaneus C, from the upper face A₁of the astragalus to a lower face C₁of the calcaneus, in such a way as to form an astragalocalcanean channel AC indicated by broken lines in <figref idref="DRAWINGS">FIG. 3A</figref>. This channel AC is drilled in a rectilinear continuation of the recess A₂, by being substantially centred on the axis A₃. To do this, the surgeon advantageously uses a drill bushing <b>50</b>, indicated partially and schematically in <figref idref="DRAWINGS">FIG. 3A</figref>. This bushing <b>50</b> has a tubular shape designed to be received in a substantially complementary manner in the recess A₂, substantially coaxial to the recess. In this way, the bushing <b>50</b> bears in the recess A₂in such a way as to guide the introduction and advance of a drill bit successively through the astragalus and the calcaneus, as is indicated by the arrow <b>51</b> in <figref idref="DRAWINGS">FIG. 3A</figref>. The surgeon thus drills the astragalocalcanean channel AC rapidly and with precision, using ancillary equipment traditional in the field of orthopaedic surgery.

After the astragalocalcanean channel AC has been drilled, the surgeon places the astragalar component <b>20</b> on the upper face A₁of the astragalus A and places the astragalocalcanean rod <b>40</b> in the channel AC. More precisely, the rod <b>40</b> is introduced into the channel AC from the lower face C₁of the calcaneus, with its end <b>41</b> directed upwards. The internal diameter of the channel AC is made substantially equal to the external diameter of the regular part <b>42</b>, such that the rod is easily advanced through the channel in an upward translational movement and parallel to the axis A₃, until the end <b>41</b> reaches the point where the channel AC opens into the recess A₂. As this recess is occupied by the stub <b>22</b>, advancing the rod <b>40</b> requires the latter to be driven in rotation on itself about the axis X-X, in such a way as to screw the external thread <b>41</b>A into the internal thread <b>22</b>A, as is indicated by the arrow <b>60</b>. In practice, the rod is driven in rotation by means of a tool whose head, indicated only by broken lines and reference number <b>61</b> in <figref idref="DRAWINGS">FIG. 3B</figref>, is engaged with the profile <b>44</b>A in the recess <b>44</b>.

It will be understood that, by giving the rod <b>40</b> a suitable lengthwise dimension, the screwing of the end <b>41</b> into the stub <b>22</b> is accompanied by the screwing of the end <b>43</b> into the calcaneus, the external thread <b>43</b>A engaging directly in the bone substance of the calcaneus at the point where the channel AC opens out on the lower face C₁of the calcaneus. In other words, the rod <b>40</b> has a length substantially equal to that of the astragalocalcanean channel AC, such that the rod can extend through the astragalus and the calcaneus with its end <b>41</b> in the area of the upper face A₁of the astragalus and with its end <b>43</b> in the area of the lower face C₁of the calcaneus.

The twin screwing of the external threads <b>41</b>A and <b>43</b>A causes the astragalus A and the calcaneus C to be gradually approximated to each other, thereby compressing these two bones against each other in the area of their respective contacting surfaces. It will be appreciated that the structural features of these external threads, in particular their respective pitch, are chosen so as to achieve compression values that are sufficient to efficiently block the astragalus and the calcaneus against each other, without generating excessive stress in the bones.

The other components of the prosthetic assembly <b>1</b> are implanted in parallel with this. The tibial component <b>10</b> is attached and immobilized on the lower end of the tibia T, with its plate <b>11</b> resting flat against the face T₁, then the prosthetic bearing <b>30</b> is attached between the components <b>10</b> and <b>20</b>. The assembly <b>1</b> is then in the implantation configuration illustrated in <figref idref="DRAWINGS">FIG. 1</figref> and the surgical intervention is concluded.

After consolidation of the arthrodesis between the astragalus A and the calcaneus C, which generally takes between six and twelve months, the astragalocalcanean rod <b>40</b> can be withdrawn by unscrewing its ends <b>41</b> and <b>43</b>. The surgeon then advantageously uses the profile <b>44</b>A by introducing a suitable disconnecting tool into the latter. To promote and accelerate this arthrodesis, bone substance can advantageously be inserted between the astragalus and the calcaneus before implantation of the prosthetic assembly <b>1</b>. Before jointly drilling right through the astragalus A and the calcaneus C, an arthroscope is inserted into the junction area between the astragalus and the calcaneus in order to place bone substance there.

<figref idref="DRAWINGS">FIG. 4</figref> shows a variant of the astragalocalcanean rod <b>40</b>, which differs from the embodiment considered in <figref idref="DRAWINGS">FIGS. 1 to 3B</figref> only at its lower end <b>43</b>. The external thread <b>43</b>A is cancelled, so that the external surface of the end <b>43</b> is smooth and extends in the straight continuation of the external surface of the regular part <b>42</b>, except at its end area that forms a bulge <b>43</b>B radially protruding externally from the rest of this surface. Unlike the embodiment of <figref idref="DRAWINGS">FIGS. 1 to 3B</figref>, the end <b>43</b> of the rod <b>40</b> of <figref idref="DRAWINGS">FIG. 4</figref> is associated with a ring <b>45</b> externally surrounding this end. This ring has an internal diameter that is both substantially equal or greater than the external diameter of end <b>43</b>, except at its end bulge <b>43</b>B, and lower than the maximum external diameter of this bulge. In practice, the ring <b>45</b> is pulled around the rod <b>40</b> by being introduced from upper end <b>41</b> while the rod is still not introduced into the astragalocalcanean channel AC.

When the external thread <b>41</b>A of the end <b>41</b> of the rod <b>40</b> is screwed into internal thread <b>22</b>A of the astragalar component <b>20</b>, by driving in rotation the rod by means of the tool whose head is engaged within the recess <b>44</b>, the ring <b>45</b> is thus interposed between the lower face C₁of the calcaneus C and the end bulge <b>43</b>B of the rod until this bulge drives the ring <b>45</b> in a pressed contact against the face C₁of the calcaneus. Thus, the ring <b>45</b> transmits and distributes the clamping stress on the face C₁of the calcaneus, in the same way as a washer. The calcaneus is thus compressed against the astragalus A, until to efficiently block the astragalus and the calcaneus against each other.

A radial clearance between the internal face of the ring <b>45</b> and the external face of the end <b>43</b>, above its end bulge <b>43</b>B, allows adjustment of the position of the ring with respect to the rod during its pressing against the calcaneus C. Thus, this relative mobility between the ring and the end <b>43</b> compensates for a potential misalignment between the central axis of the ring and the axis X-X of the rod.

According to a not shown variant, the ring <b>45</b> is fixedly secured at the lower end <b>43</b> of the rod <b>40</b>, for example being directly integral with the rod. In this case, in order to increase the pressing surface of this fixed ring against the lower face C₁of the calcaneus C during the screwing of the external thread <b>41</b>A into the astragalar component <b>20</b>, this calcanean face C₁is, if necessary, beforehand wrought, so that a substantially plane surface, inscribed in a plane substantially perpendicular to the axis A₃of the astragalocalcanean channel AC, is delimitated around the point where this channel opens into the recess A₂.

Configurations other than the external thread <b>43</b>A and the ring <b>45</b> can be provided at the lower end <b>43</b> of the astragalocalcanean rod <b>40</b> in order to engage the calcaneus C and compress it against the astragalus A when screwing the upper end <b>41</b> into the astragalar component <b>20</b>. For example, the lower end <b>43</b> can have an outer surface in the shape of a truncated cone or flare widening towards the bottom.

Other variations on the prosthetic ankle assembly <b>1</b> and on the implantation method that have been described above are also conceivable. By way of example:

The central axis of the internal thread <b>22</b>A can be offset from the central axis of the stub <b>22</b>, the drill bushing <b>50</b> then being provided with a similar offset in order to drill the astragalocalcanean channel AC in the possibly inclined continuation of the recess A₂;

The structure of internal thread <b>22</b>A and external thread <b>41</b>A can be reversed, such that the end <b>41</b> of the rod <b>40</b> forms an internal thread that can be screwed around an external thread formed in the stub <b>22</b>, for example delimited by a central pin internal to this stub; more generally, other types of means permitting coupling through complementary shapes between the upper end of the rod <b>40</b> and the lower side <b>21</b>B of the astragalar component <b>20</b> are conceivable, provided that these means lead to engagement of the lower end of the rod with the calcaneus C in order to compress the latter against the astragalus A; thus, a coupling by means of a bayonet structure is possible, and also coupling structures with stressing obtained exclusively through rectilinear translation;

By virtue of the fact that they bear slidingly on each other at their surfaces <b>11</b>A and <b>30</b>A, the tibial component <b>10</b> and the bearing <b>30</b> are movable relative to each other, which generally leads the ankle prosthesis comprising the elements <b>10</b>, <b>20</b> and <b>30</b> to be designated as a prosthesis with a “movable bearing”; however, the astragalar component <b>20</b> can alternatively be joined to a “fixed” bearing, that is to say a bearing placed fixedly against a tibial component;

The astragalar component <b>20</b> and the astragalocalcanean rod <b>40</b> are not necessarily to be used jointly with a tibial component and a prosthetic bearing such as the elements <b>10</b> and <b>30</b>; the astragalar component can in fact be made to cooperate in an articulated manner directly with the lower anatomical end of the tibia T if the latter has sufficient articular capacity; the prosthetic ankle assembly can then be designated as a partial prosthetic assembly, whereas the prosthetic assembly <b>1</b> in <figref idref="DRAWINGS">FIG. 1</figref> is to be considered as a total prosthetic assembly; and/or

If necessary, the fixation of the astragalar component <b>20</b> to the astragalus A can be strengthened by additional means.

Patents and patent applications disclosed herein, including those cited in the Background of the Invention, are hereby incorporated by reference. Other embodiments of the invention are possible. It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

In practice, the rod is introduced from a lower face of the calcaneus into an astragalocalcanean channel that has been drilled beforehand through the calcaneus and the astragalus and that opens out on the dedicated part of the astragalar component when the latter is placed on an upper face of the astragalus. Thus, advantageously in a single surgical intervention, the astragalar component and the astragalocalcanean rod are implanted by being coupled to each other during the intervention, while the rod passes right through the calcaneus and the astragalus, in order to compress them and thereby immobilize them against each other. In other words, the operation of arthroplasty between the astragalus and the tibia and the operation of arthrodesis between the astragalus and the calcaneus are performed during the same surgical intervention, which limits the surgical risks faced by the patient and which allows the surgeon to use the same surgical routes for the two operations.